1. Field of the Invention
This invention relates to a catalyst for purifying the exhaust gas from the combustion of an engine with a lean air/fuel mixture containing excess oxygen, from gas turbines such as those used in power plants, and from diesel engines. In particular, it is a durable catalyst which can remove NO.sub.x very efficiently in the presence of water and sulfur dioxide in the exhaust gas. It also relates to a method of producing such a catalyst and its end use.
2. Description of Prior Art
The current technology for pollution abatement for vehicle exhaust streams involves the application of the so-called three-way catalysts (TWC's), which have a typical composition of Pt/Rh/CeO.sub.2 /Al.sub.2 O.sub.3. The TWC's, which are able to simultaneously remove CO, hydrocarbons (HC), and NO.sub.x require approximately stoichiometric concentrations of oxidants and reductants (theoretical A/F=14.6) which may be controlled by incorporating an oxygen sensor. It is necessary for CO to be present in order to reduce NO.sub.x to N.sub.2. Hence the engine must be operated under slightly rich air/fuel ratios which is energy inefficient. The use of noble metals in the TWC is not favored because they are very expensive.
U.S. Pat. No. 5,254,519 to Rabinowitz et al. discloses a three way catalyst with a composition of rare earth oxide on a zirconia support with a rhodium component and an activated alumina support with a platinum component.
U.S. Pat. No. 5,139,992 to Dettling et al. discloses a three way catalyst for internal combustion engines.
A lean burn air/fuel mixture is desired to reduce the consumption of fuel. Excess oxygen is used to achieve high energy efficiency of the automobile engine. Selective catalytic reduction (SCR) catalysts (e.g. CuZSM-5) have been developed to remove NO.sub.x from the exhaust gas of an engine operated under lean burn conditions, but they are not sufficiently durable to meet the 100,000 mile Government requirement.
U.S. Pat. No. 5,017,538 to Takeshina et al. discloses a CuZSM-5 catalyst as a SCR catalyst to purify exhaust gas under lean burn air/fuel ratios with hydrocarbons (except methane) as reductants and its preparation method.
U.S. Pat. No. 5,260,043 to Armor et al. discloses a CoZSM-5/CoFerrierite catalyst as a SCR catalyst to purify exhaust gases under lean burn air/fuel ratio with methane as the reductant and its method of preparation.
Even though CuZSM-5, CoZSM-5 and CoFerrierite catalysts have high activity for the SCR NO.sub.x reaction, their application in industry has been limited by their low durability and poisoning by the unavoidable water present in the combustion stream. The durability of the CuZSM-5 catalyst in the presence of water and SO.sub.2 was evaluated by scientists and engineers in General Motor Research Labs. As a result, the CuZSM-5 catalyst was found to have very poor thermal stability (lost 80% of its initial activity in 4 hours). Monroe, D. R. et al. SAE Report No. 930737, 1992. The durability study of CoZSM-5 and CoFerrierite catalysts was performed by scientists at Air Product Co., and the research showed a substantial decrease in the NO.sub.x conversion in the presence of water and SO.sub.2. Armor, J. N. Catal. Today 26, 147, 1995. The most recent reviews in the area of NO.sub.x SCR claim that there is no catalyst which has both the required de-NO.sub.x activity and durability (against water and SO.sub.2) for any practical usage. Walker, A. P., Catal. Today 26, 107, 1995.b) Armor, J. N. Catal. Today 26, 99, 1995. There are two reasons for this: a) a reversible poisoning by H.sub.2 O; and b) an irreversible breakdown of structure due to dealumination leading to hydrothermal sintering. CuZSM-5, CoZSM-5 and CoFerrierite deactivate very rapidly if H.sub.2 O or SO.sub.2 or both are present in the reaction stream. This is unavoidable in any combustion stream. With presently available technology, there is no way to prevent deactivation of CuZSM-5, CoZSM-5 and CoFerrierite catalysts.
The above catalysts are zeolites (crystalline aluminosilicates) that may be represented by the general formula: EQU x/nM.sup.+.spsp.n [AlO.sub.2 ].sub.x [SiO.sub.2 ].sub.y
Many zeolites which have different crystal structures, channel openings, and one or three dimensional arrangement of these channels are commercially available. The cation (M) exchange capacity depends on the tetrahedral aluminum content which creates negative charge on the framework of zeolite that must be compensated by cations, such as H.sup.+ Na.sup.+, K.sup.+, Cu.sup.2+, Fe.sup.2+, Co.sup.+2, Co.sup.+3, CuOH.sup.+, and FeOH.sup.+ and many others.
However, the uniform crystal structure of zeolite is subject to attack by molecular water which eventually will break down the crystal structure of zeolite by breaking and reforming T--O--T bonds (T=Al, or Si). This break-down of the zeolite's structure removes tetrahedral aluminum from the lattice lowering its base-exchange capacity and destabilizing the lattice, destroys the porosity and channel framework, and deactivates the catalyst. It has been suggested in the art that the quick deactivation of CuZSM-5 catalyst in the presence of H.sub.2 O is due to the combination of a) dealumination by steaming, b) zeolite crystallinity break down, and c) poisoning of the active sites by H.sub.2 O and/or SO.sub.2.
Therefore, in spite of these prior art disclosures, there remains a very real and substantial need for an internal combustion engine and power plant exhaust catalyst and process of making such catalyst that when used for a lean air/fuel mixture, it can remove NO.sub.x substantially with the same efficiency irrespective of the air-fuel ratio of the lean fuel mixture. There is also a great need for a catalyst with high-stability and durability against deactivation by water and sulfur dioxide during use.